1. Field of the Invention
The present invention relates to a process for preparing a carboxypolysaccharide by the oxidation of a polysaccharide. The carboxypolysaccharide which can be obtained by the present invention can be utilized as any of a scale deposition inhibitor, a pigment dispersant, a sizing agent, a concrete blending agent, a detergent builder and the like.
2. Description of the Related Art
Heretofore, acrylic acid polymers and acrylic acidmaleic acid copolymers have been industrially been manufactured as detergent builders. However, these synthetic polycarboxylic acids comprise a vinyl polymer structure which is not general as a natural polymeric chemical structure, and hence, it is known that there is a problem that the biodegradability of the synthetic polycarboxylic acids by microorganisms is very low. On the other hand, a polycarboxylic acid prepared by the oxidative carboxylation of a polysaccharide has the high biodegradability, and it can be expected that this polycarboxylic acid is useful to solve the above problem as a biodegradable builder.
Various methods for preparing a carboxypolysacchaide by oxidizing the polysaccharide are known so far. For example, Japanese Patent Publication No. 1281/1974 discloses a method for oxidizing the polysaccharide by the use of a combination of periodic acid and a chlorite, or a hypochlorite, and it is further described herein that a dicarboxystarch obtained by oxidizing the C.sub.2 and C.sub.3 positions of a monosaccharide which is a constitutional unit of the polysaccharide has a detergent builder function. As methods for preparing a tricarboxypolysaccharide by oxidizing the polysaccharide, techniques which comprises treating starch with a periodate to form an aldehyde, and then carrying out the oxidative carboxylation of the aldehyde with dinitrogen tetraoxide are disclosed in Japanese Patent Publication No. 40552/1972 and Czechoslovakia Patent No. 235576. In these methods described in these publications, however, the expensive periodate needs to be used, and for this reason, these known methods are not industrially desirable.
Japanese Patent Application Laid-open No. 71601/1997 discloses a process for preparing a tricarboxystarch by carboxylating starch with a hypochlorite as an oxidizing agent in the presence of a ruthenium catalyst. Since ruthenium which is used herein is an extremely expensive transition metal, the loss of ruthenium is severely inhibited in a tricarboxystarch manufacturing process. According to the investigation by the present inventors, however, the tricarboxystarch produced by the above method contains a certain amount of ruthenium, which means the loss of ruthenium, and moreover, there is a problem that an operation of removing ruthenium from the produced tricarboxystarch is required.
As techniques of removing ruthenium from the tricarboxypolysaccharide produced by oxidizing a polysaccharide such as starch, there are a method which comprises oxidizing the ruthenium catalyst to form ruthenium tetraoxide, and then eliminating it by distillation or extraction with an organic solvent, a method which comprises adsorbing the ruthenium catalyst on an organic or an inorganic carrier, and a method which comprises extracting the ruthenium catalyst at ordinary temperature with a chelating agent. However, the certain amount of ruthenium contained in the tricarboxypolysaccharide is inactive in the above removal operation, so that it has been extremely difficult to lower the ruthenium content contained in the tricarboxypolysaccharide to less than 100 ppm.